To sample the current of a multi-mode circuit, a traditional method is to add an external resistance in parallel. As shown in FIG. 1, a traditional current sampling circuit applied to a multi-mode circuit comprises a first MOSFET (Metal Oxide Semiconductor Field Effect Transistor) SWA, a second MOSFET SWB, a third MOSFET SWC, a fourth MOSFET SWD, an inductance L and a sampling resistance Rs, wherein the sampling resistance Rs is coupled to the drain of the fourth MOSFET SWD. An output current Iout will be obtained by calculating the voltage Vs across the sampling resistance Rs.
However, for the external sampling resistance Rs, energy is dissipated and lost, and two extra pins are needed to couple the sampling resistance Rs, so the cost is also increased.